The invention pertains to internal combustion engines, and in particular, to four cycle internal combustion engines operating on the Otto cycle or the Diesel cycle, the former being a constant volume combustion cycle and the latter being a limited pressure combustion cycle.
There are several variations to these basic cycles. Two that are of particular importance are the Atkinson cycle and the Miller cycle. In the basic theory of a naturally aspirated Otto cycle engine, compression occurs from bottom dead center to top dead center with the valves closed. Ignition occurs at top dead center. With ignition the pressure within the cylinder increases and the piston retreats back to bottom dead center. The exhaust valve opens and the exhaust is dumped as the piston again moves to top dead center. The exhaust valve closes and the intake valve opens with the piston now moving back toward bottom dead center to draw in a fresh charge.
In an actual engine, the theoretical valve timing of the Otto cycle is modified substantially to take advantage of inertia effects of the moving intake air and exhaust gases. In an actual engine, the intake valve is closed after bottom dead center and during the initial portion of the compression stroke to take advantage of the inertia effects of the intake air and thereby increase the trapped intake charge resulting in higher engine power. Similarly, the exhaust valve is open before bottom dead center allowing exhaust blow down and thereby returning the cylinder pressure to near atmospheric before the piston moves from bottom dead center to top dead center pushing the exhaust from the cylinder. Slightly before top dead center the intake valve opens to allow the intake charge to begin flowing and slightly after top dead center, the exhaust valve closes. In the naturally aspirated Otto cycle engine, the cylinder pressure within the cylinder during the intake stroke is normally below atmospheric pressure. However, at wide open throttle and high speed, toward the end of the intake stroke, the pressure within the cylinder of a well designed engine may exceed atmospheric pressure due to inertial effects.
Turbo supercharging of the Otto cycle engine assures that the cylinder pressure during the intake stroke from top dead center to bottom dead center is always above atmospheric pressure and the intake manifold pressure is almost always above the cylinder pressure during the exhaust stroke from bottom dead center to top dead center. Only at extremely low speeds is the cylinder pressure of the exhaust cycle likely to equal or exceed the intake manifold pressure. The cylinder pressure during the exhaust stroke from bottom dead center to top dead center is greater than atmospheric pressure.
The naturally aspirated Diesel cycle engine operates substantially similar to the naturally aspirated Otto cycle, however, the constant volume burning at top dead center is followed by constant pressure burning as the piston descends toward bottom dead center on the expansion stroke. During intake from top dead center to bottom dead center, the cylinder pressure during the intake is closer to atmospheric at all loads because the naturally aspirated Diesel engine does not utilize throttling for the intake air. With turbo charging of the Diesel engine, the cylinder pressures during the intake and exhaust strokes of the diesel engine behave in a manner similar to the cylinder pressures of the turbocharged Otto cycle engine.
The Atkinson cycle comprises a modification to either the Otto cycle or Diesel cycle. The Atkinson cycle comprises a cycle in which the expansion stroke is much longer and larger than the compression stroke (Combustion Engine Processes, Lester C. Lichty, 1967, McGraw-Hill p. 10). In the true Atkinson cycle engine, a special crank shaft linkage causes the expansion stroke to be longer than the compression stroke. In the modified Atkinson engine, the intake valve closing is either substantially earlier or substantially later than otherwise, either of which leads to an artificially shortened compression stroke (Effects of Intake-Valve Closing Timing On Spark-Ignition Engine Combustion, SAE 850074).
The Miller cycle can also be applied to the Otto cycle or Diesel cycle engine and borrows the Atkinson cycle principle of a larger expansion stroke than compression stroke (A New Type Of Miller Supercharging System For High Speed Engines Part 2--Realization Of High BMEP Diesel Engines, SAE 851523). In addition, the Miller cycle consists of an increased charging pressure over that feasible without the use of the Atkinson principle and a variation in the intake valve timing while the engine is running.
In essence, the Miller cycle is directed to shifting the intake valve closing to an earlier time before bottom dead center as the load on the engine increases. To compensate for the decreased intake flow because of the early closure of the intake valve, the boost pressure on the turbocharger is increased to provide an intake air charge of essentially the same mass. The pressure volume diagram of the Miller cycle appears like a turbocharged Otto or Diesel cycle engine with the early intake valve closing of the Atkinson cycle. In order to achieve the high levels of turbocharger boost necessary to operate the engine on the Miller cycle, most such engines use a two-stage turbocharger and usually include air coolers for the intake charge. The primary purpose of the Miller cycle is to increase thermal efficiency while maintaining high specific output through high boost pressure.
However, the complex mechanical components necessary to vary the valve timing have lead to the following variants of the Miller cycle. First, is the auxiliary intake control rotary valve (ICRV) (A New Type Of Miller Supercharging System For High Speed Engines Part 1--Fundamental Considerations And Application To Gasoline Engines, SAE 851522, SAE 851523).
In the ICRV concept, an auxiliary rotary control valve is positioned upstream of a normal intake valve. The timing of the intake valve is near bottom dead center. The timing of the rotary valve is adjusted while running to close off the intake channel prior to the closing of the intake valve. The closed timing of the rotary valve is dependent on speed and load (boost pressure) which results in a pressure volume relationship that simulates a normal Miller cycle engine (SAE 851522, p. 3).
The Miller cycle engine suggests that the intake valve can be closed before or near bottom dead center and power can be maintained by utilizing extremely high boost pressures (3.5-5.5 bar) (A New Type Of Miller Cycle Diesel Engines, Sakai et. al. p. 1 & FIG. 6, JSAE Vol. 9, no. 2, Apr. 19, 1988). Thus, the principal of the Miller cycle is to increase the charge density without increasing the maximum pressure in the cylinder. The ratio of the exhaust back pressure to the inlet boost pressure for maximum efficiency should be close to 1; however, in practice, 0.67 is normally used (The Internal Combustion Engine In Theory And Practice, Vol. 1: Thermodynamics, Fluid Flow, Performance, Charles F. Taylor, Fifth Printing Second Edition 1982, MIT Press, p. 384 and example 10-4).
The other variant of the Miller cycle for Diesel engines is provided by the exhaust leak-down method disclosed in U.S. Pat. No. 4,424,790. In the exhaust leak-down method, the intake valve closes near bottom dead center. The exhaust leak-down concept controls the cylinder pressure, and hence the compression stroke by bleeding off cylinder pressure by two alternate and equivalent means. One means is to hold the exhaust valve slightly open throughout the intake stroke. In this way, a portion of the boost pressure is continually blown out the exhaust valve. In the other approach, the exhaust valve is opened immediately after the intake valve has closed allowing the cylinder pressure to escape through the exhaust valve. The amount of pressure bled off the cylinder is automatically controlled by the difference between the boost pressure provided by the turbocharger required in the Miller cycle and the exhaust back pressure created by the turbocharger. As a result, the pressure-volume relationship is modified from the Miller cycle by moving the intake cylinder pressure from the boost pressure towards the average steady flow exhaust pressure (as defined by Taylor, p. 382) during the time the auxiliary exhaust leak is opened.
U.S. Pat. No. 4,424,790 thus discloses an exhaust pressure modulated bleed-off of the boost pressure to achieve a Miller cycle engine. This patent shows that volumetric efficiency and trapping efficiency for the cycle disclosed therein, the Miller cycle, and the Atkinson cycle go down with increasing load. Further, the reference claims that holding the exhaust valve partially open throughout the intake cycle is equivalent to closing the exhaust at the normal time and then reopening the exhaust valve after the intake valve is closed.
To summarize, the momentum effects of a high-speed four cycle Otto or Diesel cycle engine that does not employ the Miller or Atkinson principles, must, in the valve timing, delay the closing of the intake substantially beyond bottom dead center to obtain reasonable power. Generally, the higher the speed or the higher the specific output desired, the later intake valve closing occurs (Taylor, p. 193; Internal Combustion Engines And Air Pollution, Edward F. Oberth, Harper & Row 1973, p. 471-474).
Empirically observed pressure waves or pulses in exhaust pipes are discussed along with computer simulations in a publication entitled "Gas Flow in the Internal Combustion Engine", W. J. D. Annand and G. E. Roe, G. T. Foulis & Co., Ltd., 1974, Sparkford, Yeovil, Somerset, England. Discussed are compression, rarefaction and reflected compression waves. As described, the compression wave is a positive pressure wave occurring when a valve opens and the high upstream cylinder pressure escapes into the exhaust pipe or system. This phenomenon is described by Taylor (p. 382) in terms of the blow-down portion of the exhaust cycle. The rarefaction wave described by Annand and Roe is a "negative pressure" wave transmitted upstream. A reflected compression wave is also described and is a positive pressure wave transmitted upstream which is commonly termed a "plugging pulse". The plugging pulse can prevent the overshoot of intake gases into the exhaust pipe at the end of scavenging during valve overlap (four cycle engine) or the loss of cylinder pressure (two cycle engine).
Annand, et al. describe the effect of the exhaust pipe geometry on the timing and magnitude of compression and rarefaction waves in the exhaust system. The geometry discussed includes constant diameter (straight pipe), divergent, and expansion box pipes. The expansion box disclosed includes a divergent section followed by a convergent section usually with a constant diameter section therebetween. Both the divergent exhaust pipe and the expansion box have a smooth transition between the upstream exhaust pipe (usually constant diameter) and the entrance to the divergent section.
According to Annand, et al. in the straight pipe a rarefaction wave occurs from the sudden expansion of gases at the end of the pipe, the rarefaction wave then traveling upstream in the pipe. Generally an almost imperceptible reflected compression wave ("plugging pulse") also occurs from a straight pipe. In the divergent pipe the rarefaction wave is stronger and more ordered than in the straight pipe. The reflected compression wave also appears as a small but ordered plugging pulse.
The expansion box through the divergent section also produces rarefaction waves moving upstream, however, due to the geometry of the divergent-convergent sections, the strength and general order of the reflected compression wave (plugging pulse) transmitted upstream is much greater than in either the straight pipe or the divergent pipe.
Four cycle engines predominately use an open end constant diameter exhaust pipe or straight pipe. Very rarely is a divergent exhaust pipe used with a four cycle engine, and the expansion box per se is never applied to a four cycle engine. In a four cycle engine with a straight pipe the blow down pulse creates a rarefaction wave that travels upstream and can cause a lower pressure in the cylinder during the exhaust stroke. In a well designed exhaust system, the cylinder pressure during the exhaust stroke can be below atmospheric pressure which increases the scavenging of the exhaust from the cylinder. Subsequently, a pressure wave or plugging pulse is reflected upstream, in particular, when multiple straight pipes are connected to a pulse convertor or collector. In a well designed system, the plugging pulse will arrive at the cylinder in time to prevent the intake charge from exiting through the exhaust valve to the exhaust pipe system. During the early portion of the intake stroke, both the intake and exhaust valve are simultaneously opened (valve overlap) and absent the plugging pulse, the intake charge can exit into the exhaust system.
In contrast, two cycle engines use the plugging pulse or reflected pressure wave in an entirely different manner for a different purpose. The exhaust system of a two cycle engine is typically designed to draw a significant amount of the intake charge into the exhaust system (which includes an expansion box) due to the rarefaction wave following the onset of the blowdown period, since at the time both the exhaust ports and the intake transfer ports are open. Properly designed, the expansion box causes the reflected pressure wave to force the re-entry of some of the intake charge drawn into the expansion box back into the cylinder after the intake transfer ports or passageway leading thereto are closed thereby increasing the trapped charge.
The current approach and design of an expansion box for a two cycle engine is not simply transferable to a four cycle engine, the reason being the reflected pressure wave within the expansion box arrives much too early for the proper effect since the four cycle engine is still on the exhaust stroke. Furthermore, if the reflected pressure wave is delayed to arrive at an equivalent point in the compression stroke of a four cycle engine as compared to a two cycle engine, then the reflected pressure wave arrives to impact the closed exhaust valve. A recent article (Circle Track Magazine, P. Saueracker, January 1989, pp. 68-72) discusses why a reflected compression wave is very undesirable and detrimental to power production in a four cycle engine. Saueracker further describes various techniques that automobile racers are currently using to prevent or attenuate the reflected compression wave as it moves upstream.
Most common are stepped design exhaust headers comprising a series of different diameter straight pipes with sudden transitions between pipes, the pipes decreasing in diameter in the upstream direction. Another approach comprises an anti-reflection chamber ("anti-reversionary chamber") having a diverging section joined to a converging section by an outer constant diameter pipe. But, unlike an expansion box, in the anti-reflection chamber the inlet exhaust pipe extends at constant diameter substantially into the chamber. As taught by Saueracker a zone between the extended internal pipe and the divergent section traps the reflected compression wave before further movement upstream.
U.S. Pat. No. 1,952,881 discloses method and apparatus for reintroducing exhaust gases into the combustion chamber of an engine. The reintroduction of the exhaust gases is accomplished by modifying the cam for the exhaust valve to retain the exhaust valve open during the period of time that the cylinder pressure is less than the pressure in a common log exhaust manifold of a multicylinder engine, thereby allowing the exhaust gases to flow back into the cylinder from the common log manifold. The purpose for the reintroduction of the exhaust gases is to reduce detonation in the cylinder thereby lowering the required octane value of the fuel.
Another relatively early U.S. Pat. No. 2,131,958 discloses a two cycle fuel injected engine cylinder equipped with a crank shaft driven rotary exhaust valve downstream of the exhaust port and a reflection delay device comprising an expansion box with an internal swirl generator. This disclosure is directed to the reintroduction of exhaust gases into the cylinder by the timing of the rotary valve and by additionally delaying the reflected pressure pulse in the exhaust system.
U.S. Pat. No. 2,476,816 discloses two cycle multi-cylinder engines with an exhaust manifold system of constant diameter pipes configured to cause the pressure wave or pressure pulse of blowdown from one cylinder to arrive at another cylinder just prior to the closure of the exhaust at the second cylinder. The result is a sudden charging pressure introduced into the second cylinder just prior to the closure of the exhaust port to the second cylinder. The result is claimed to be more effective in a multi-cylinder two cycle engine than attempting to reintroduce the reflected pressure pulse from a constant diameter pipe into the same cylinder.
More recently, U.S. Pat. No. 3,298,332 disclosed the application of the above blowdown pressure pulse with constant diameter pipes in a multi-cylinder engine to a four cycle engine. The exhaust system disclosed is used in conjunction with a specified intake system consisting of a series of constant diameter pipes. Furthermore, the exhaust system disclosed is limited to connection of cylinders that are 360.degree. apart in the firing order. Thus the blowdown pulse from one cylinder travels through the mutually connected exhaust system so configured that the pulse arrives at the exhaust valve of second cylinder 360.degree. away just prior to the closure of the second cylinder. The disclosure does include the re-entry of any intake air that has escaped into the exhaust manifold thereby improving the volumetric efficiency of the engine.